Wireless telecommunication base stations in the prior art typically employ one or more broad beam antennas to transmit and receive information-bearing signals to wireless terminals, e.g., cellular telephones, within a given radius of the base station. With this approach, each broad beam in azimuth can typically be used as the conduit for communications with a mobile wireless terminal traveling over a wide angular sector in relation to the base station.
One shortcoming of broad beam systems is short communication range. In general, the distance from which wireless terminals can communicate with the base station is proportional to the base station antenna gain, and inversely proportional to the solid angle beamwidth, for a given transmitter power level. More specifically, the nth power of this distance is directly proportional to the base station antenna gain, where n is typically between three and four. Short communication range can pose severe cost constraints for practical systems, especially in rural areas, since a large number of base stations would be needed to cover a correspondingly large geographical area.
Another drawback of broad beam systems is inefficient use of transmitter power. For example, in frequency-division-multiple-access (FDMA) systems, a narrow frequency channel, e.g., 30 kHz wide, is dedicated to each communication session with a wireless terminal. Typically, each broad beam transmits a frequency division multiplexed (FDM) signal carrying information-bearing signals of many communication channels to associated wireless terminals. Since each wireless terminal is located in only one angular direction at a time, and signal power intended for that wireless terminal is transmitted over a broad angular sector, power is wasted. In addition, the likelihood of interference from other communication sessions can be high, inasmuch as each wireless terminal must filter out many undesired signals in order to communicate. Interference oftentimes results when such filtering is less than perfect. The base station receiver is faced with an analogous problem.
One solution to the above-noted limitations of broad beam systems is disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 08/506286 entitled "Power Shared Linear Amplifier Network". Therein, multi-beam communication systems are disclosed, each of which utilize a power-shared amplifier network comprising a pair of power sharing networks, such as Butler Matrices, and a plurality of amplifiers coupled therebetween. The power sharing aspect of the system provides advantages over compartmentalized narrow beam systems that use a power amplifier dedicated to each beam. That is, each amplifier in the power shared amplifier network amplifies power of all the information-bearing signals to be transmitted, thereby improving system reliability and enhancing communication traffic-handling capability on a statistical basis. Multiple, high gain beams can be formed either by a separate antenna dedicated for each beam, or by several multi-beam antennas. The disclosed systems, however, typically utilize switching networks to switch modulated signals to the appropriate antenna beam for transmission and reception. For systems with a large number of users, such switching networks can be become complex and costly. Additionally, if the base station radios used for modulating the information-bearing signals are located at the bottom of a base station tower, and the antennas are at the top of the tower, as is typical, the cabling requirements to connect the radios to the antennas can become burdensome.